poll: changed to include <poll.h> instead of <sys/poll.h>

This is a very cosmetic change but I think it is good to be
standards compliant. The standard defines <poll.h> and not
<sys/poll.h>:

http://pubs.opengroup.org/onlinepubs/009696799/basedefs/poll.h.html

When building against musl libc it silences some annoying cpp
warnings like this:

-----------------------------------------------------------------------
warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
-----------------------------------------------------------------------

Signed-off-by: Christian Wiese <chris@opensde.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>